1. Field of the Invention
The present invention relates to an apparatus for preventing bulging of both edges of a strip while preventing contamination of a roll surface in a twin roll strip caster. More particularly, the apparatus of the invention mutually adjusts gas pressure in a first space between weirs for preventing molten level fluctuation and gas pressure in second spaces outside the weirs where rolls contact with molten steel so that metal components evaporated from a surface of molten steel may not stick to roll edges, and outwardly exhausts the metal components evaporated in the second spaces mainly through a central portion rather than through edge portions in respect to a roll lateral direction so as to realize a defectless strip through uniform condensation in the roll lateral direction.
2. Description of the Prior Art
As shown in FIG. 1, a conventional twin roll caster 100 feeds molten steel from a tundish 2 through an immersion nozzle 3 into a space between dams 4 closely contacted to both sides of casting rolls 1 and 1a so that a molten iron pool 13 is formed by the casting rolls 1 and 1a and the edge dams 4. A meniscus shield 5 is installed over the casting rolls 1 and 1a to prevent oxidation of molten steel owing to oxygen contained in air contacting with a surface of the molten iron pool 13, and non-oxidizing gas is fed into a hollow space between an underside of the meniscus shield 5 and the surface of the molten steel pool 13. As a result, the surface of the molten steel pool 13 contacts with non-oxidizing gas thereby restraining oxidation of molten metal by the maximum amount.
Japanese Laid-Open Patent Application Ser. No. H6-297111 proposes a sealing apparatus, which is arranged over molten steel level to adjust the depth of the immersion nozzle 3 immersed into molten steel while shielding the molten steel pool 13 surrounded by the casting rolls 1 and 1a and the edge dams 4 from an external oxidizing atmosphere in order to prevent oxidation of molten steel in the molten steel pool 13.
According to the above document, where it is necessary to vary molten steel level during casting, adjustment of a spring mounted on a lateral portion of the tundish can regulate the immersed depth of the immersion nozzle 3 while successively maintaining the atmosphere over the molten steel surface.
Furthermore, a Japanese Laid-Open Patent Application Ser. No. H7-204795 is aimed to install side dams or weirs which are partially immersed into the molten steel pool 13 so as to prevent oxide created in the molten steel surface from flowing into a solidification cell mixed with molten steel. That is, this technology sets a certain gap between each of the casting rolls 1 and 1a and each of the side dams and maintains the molten steel level within this gap and a space over molten steel in a non-oxidizing gas atmosphere so as to prevent creation of oxide by the maximum amount as well as created oxide from flowing into the growing solidification cell mixed with molten steel.
However, even though the non-oxidizing atmosphere gas is filled over the molten steel surface, volatile components such as Mn, Zn and Pb escapes from the molten steel pool 13 into the atmosphere gas so as to mix with pure non-oxidizing gas thereby creating contaminated gas.
Since contaminated gas contains the above-mentioned volatile components, it is condensed in contact with cold surfaces of the casting rolls 1 and 1a and thus sticks thereto to influence heat transmission of the casting rolls 1 and 1a thereby deteriorating the quality of a strip 10. In order to avoid this problem, it is necessary to avoid the surfaces of the casting rolls 1 and 1a from contacting with contaminated gas if possible.
Furthermore, since this technology has no means for controlling the flow of volatile metal gas created from the molten steel surface, such volatile metal gas or contaminated gas contaminates the surfaces of the casting rolls 1 and 1a thereby causing surface defects of the strip 10 as well as degrading productivity.
Gas pipes 6 are installed at both sides of the meniscus shield 5 parallel to the longitudinal direction of the rolls and connected to a gas feeding line 9 for feeding non-oxidizing gas to block external gas from flowing into a lower space of the meniscus shield 5. A second flow of non-oxidizing gas such as nitrogen is also injected toward outer peripheral faces of the rolls to prevent external air from flowing into the lower space of the meniscus shield as well as assist the above-mentioned flow of non-oxidizing gas.
Molten steel dispensed via a nozzle hole 14 in a lower end of the immersion nozzle 3 may create molten level fluctuations since the flowing molten steel has a very large amount of momentum. In order to regulate molten steel fluctuation in the molten steel pool 13, weirs 12 are installed along a longitudinal direction of the rolls with their upper ends fixed to weir supports 17, see FIG. 2.
Although it is most preferred to maintain the non-oxidizing atmosphere in the space over the surface of the molten steel pool 13, the space is rarelyprovided with perfect sealing and oxide is partially produced in the surface of the molten steel. The weirs 12 act as barriers to prevent the above-produced oxide from reaching the growing solidification cell.
In the molten steel pool 13 formed between the casting rolls 1 and 1a, specific substances such as Mn continuously evaporate from the surface of molten steel in the lower space of the meniscus shield 5, and the above-mentioned metal components or volatile impurities mix with non-oxidizing gas fed onto the molten steel surface moving along the non-oxidizing gas flow.
Since the volatilized substances such as Mn have a very low value of thermal conductivity, they may function as a thermal resistance when deposited on the surfaces of the casting rolls 1 and 1a in formation of a solidified cell 11, thereby creating regional bulging owing to non-solidification and resultant defects of the strip 10.
Natural exhaustion is generally made in the space under the meniscus shield 5. However, when contaminated gas is produced exceeding a reference quantity, it is necessary to actuate a gas exhaust hose 8 via an exhaust pump 7 to adjust the quantity of gas which is exhausted to the outside.
As shown in FIG. 2, the flow of non-oxidizing gas is produced via a gap between the edge dam 4 and the weir 12. When the gap between the weir 12 and the edge dam 4 is removed to clear the flow of non-oxidizing gas, that is, the weir 12 closely contacts with the edge dam 4, skull is created in a contacting region between the edge dam 4 and the weir 12 so that the weir 12 may be damaged in some hostile situations thereby suspending casting.
Further, the edge dam 4 is sometimes vibrated in order to restrain skull creation on a surface of the edge dam 4. However, if the weir 12 is closely contacted with the edge dam 4, vibration acting on the edge dam 4 may damage the weir 12. Accordingly, it is not preferred to closely contact the edge dam 4 with the weir 12.
As set forth above, non-oxidizing gas externally supplied into the meniscus shield 5 is contaminated through mixture with the evaporated metal components from the molten steel pool 13 while flowing through the meniscus shield 5. When mixed gas or contaminated gas contacts with the surfaces of the casting rolls 1 and 1a, the evaporated metal components are condensed into solid again to stick to the surfaces of the casting rolls 1 and 1a. 
The evaporated solid components stuck to the casting rolls 1 and 1a obstruct heat transmission of the casting rolls 1 and 1a and thus vary the thickness of the adjacent solidification cell 11, thereby causing cracks in solidification of a cast strip. As a result, it is necessary to manage the casting rolls 1 and 1a in such a manner that contaminated gas does not stick to the surfaces of the casting rolls 1 and 1a. 
The present invention is intended to solve the foregoing problems of the prior art by providing an apparatus for preventing bulging of both edges of a strip while preventing contamination of a roll surface in a twin roll strip caster, which prevents contaminated gas formed through mixture of non-oxidizing gas and the evaporated metal components from contacting and sticking to the roll surfaces or mixing into molten steel.